This invention relates to a device for transmission of information between a plurality of data acquisition devices and a central recording device.
More particularly, the transmission system according to the invention is adapted to transmit to a central recording device, located on a ship, seismic informations, collected from hydrophones of a seismic streamer of great length and previously gathered in a certain number of data acquisition devices distributed over the streamer.
The marine seismic prospecting methods generally comprise the transmission of acoustic waves through water, the reception of echoes of these seismic waves, after reflection on various reflectors or mirrors of the submerged formations, and their recording. The reception of echoes is performed by means of a generally high number of interconnected hydrophones or hydrophone groups, arranged at regulator intervals along an elongate seismic streamer towed behind the ship. Each interconnected hydrophone or hydrophone group is connected, through a pair of conductors, to a single data recording system located on the ship. The recorded signals emanating from each hydrophone group, form a trace. The seismic streamers generally consist of elements connected to one another through detachable connection systems comprising connectors for electrically interconnecting the different conductor pairs, the assembly being connected to the ship.
The most recent seismic methods comprise the use of more and more numerous hydrophones distributed along seismic streamers of an increasing length which may reach three or four kilometers in length. The number of hydrophone groups forming the seismic streamer may reach 500. The use of a reception device of such a great length enables to increasing the resolution power, i.e. the ability to differentiate two adjacent geologic layers of the system. Moreover, when a substantial number of recording traces is available, it is possible, in a later processing stage, to effect a number of various different combinations by varying the number of traces used in these combinations and, consequently, the configuration of the assembly of the hydrophone groups producing the combined signals. Thereby are avoided the changes which must be made in the geometry of less complex seismic streamers in order to proceed to certain combinations, in view of the more limited number of recording traces which can be combined.
The number of conductors to interconnect through connectors in seismic streamers of great length comprising a large number of hydrophones, may be very substantial. In the example of a seismic streamer designed to give 500 traces, it is necessary to make use of connectors with 1000 pins. It is easily understood that the reliability of such seismic data transmission systems quickly becomes questionable when the number of traces to be recorded increases.
A known process for simplifying the information transmission facilities consists of making use of information analog multiplexing. The hydrophones or hydrophone groups are sequentially connected through switches to a single pair of connectors. The transmitted information consists of a sequence of analog samples successively delivered by hydrophone groups. However, this process does not give satisfactory results since diaphony between the signal samples is much too high.
Another known process for avoiding the disadvantages due to the analog multiplexing consists of making use of multiplexing of digital type. Inside the reception device are arranged interconnected data acquisition assemblies spaced from one another. Each acquisition assembly is adapted to convert the analog signals produced by an assembly of hydrophones or hydrophone groups distributed over a section of a seismic streamer to digital signals and to store them in a memorizing member.
A first mode for connecting the data acquisition assemblies to the central recording system consists of a parallel connection on two connecting omnibus cables. The recording system successively transmits interrogation signals, through one of the omnibus cables, to the different data acquisition assemblies, and orders them to sequentially connect themselves onto the other omnibus cable and to transmit thereto the digitized data recorded therein after the preliminary emission of recognition signals to acknowledge reception of the transmitted orders. The transmission of the recorded data is preceded or followed with that of service signals which, by increasing the rate of information to be transmitted, makes necessary a widening of the bandwith. However, this bandwith tends to become more and more damaged as the length of the omnibus cables on which are connected the data acquisition systems increases up to a length which may sometimes reach several kilometers.
In addition, as the data acquisition systems are not disconnectable, the malfunctioning of one of them may deform the information transmitted from the others.
According to another arrangement, the transmission device comprises a plurality of information amplification and/or regeneration members (usually called "repeaters" or repeater-regenerators) which are serially interconnected to a single transmission line.
Some of them comprise switching means adapted to connect the following length section with either the preceeding line section or with a data acquisition assembly. Control means, located on the ship, enables, by conveniently actuating the different switching means, to successively connect the different data acquisition assemblies. The transmission of the information is performed serially in a synchronous manner. It must be observed that with such a transmission device, the malfunctioning of one amplification and/or regeneration member may be sufficient to stop any information transmission. This disadvantage is particularly troublesome in marine seismic prospecting since repairs require the rewinding of the seismic streamer.
According to another arrangement, the various data acquisition devices are connected to the central recording device through at least three different transmission lines and the informations are transmitted on these three lines. The central recording device makes comparisons between the simultaneously received informations to select therefrom those which have been correctly transmitted and to detect eventual transmission errors. The tests are thus conducted simultaneously with the transmission of the seismic informations.